Although batch processing as a method of recycling asphaltic concrete has been practiced, such a process has not been satisfactory, primarily due to the substantial limitation of the amount of used or reclaimed asphaltic concrete that can be incorporated in the final product. In that method, the asphaltic concrete to be recycled is mixed with super heated virgin aggregate to produce the recycle mix. Because the used asphaltic concrete is not heated prior to being mixed with the hot aggregate, the latter must be heated to very high temperatures in order to achieve a recycle mix having adequately high temperatures for use, generally preferably above about 225.degree. F., and more preferably above about 250.degree. F. For many practical uses, it is desired to have mix temperatures of around 275.degree.-300.degree. F. or higher, so it becomes apparent in order to achieve such temperatures, substantial super heating of the virgin aggregate must be accomplished.
Where small amounts of the reclaimed material are to be used in the recycle mix, to achieve a product in the desired temperature range, virgin aggregate super heating in the 300.degree. F. range may be suitable. However, where it is desirable to include as much reclaimed material as possible in the final product, much higher aggregate temperatures are required. Indeed, heretofore, because of such limitations, the maximum amount of reclaimed material which it has been possible to use in the recycled mix in a batch process of this type has been reported to be about 50%. For example, for a final 225.degree. F. mix temperature, assuming zero moisture in the reclaimed material and ambient material temperature of 50.degree. F., virgin aggregate temperatures of about 400.degree. F. are required, whereas at about 4% moisture, aggregate temperatures must be about 600.degree. F., assuming no heat loss during processing and mixing. For final recycle mix temperatures of 260.degree. F., with zero moisture in the reclaimed material, virgin aggregate temperatures of almost 500.degree. are required; with 2% moisture, the aggregate must be super heated to almost 600.degree. F., and at 4% moisture, to about 675.degree. . Obviously, such extreme aggregate temperatures present significant problems in heating the material in conventional dryer drums and in handling the material before it is mixed with the ambient reclaimed material. Moreover, with aggregate temperatures of above about 300.degree. F., there are also substantial problems of asphalt deterioration and burning as the super hot aggregate is initially exposed to the asphalt present in the reclaimed material.
Attempts to avoid such problems in batch processing have not been accomplished heretofore because of the inherent limitations in heating the reclaimed material by direct exposure to hot gases of combustion in conventional dryer drum apparatus. To specifically avoid burning the asphalt in the reclaimed material, alternate methods have been proposed such as described in my U.S. Pat. No. 4,177,080 and those of others such as U.S. Pat. Nos. 4,075,710 and 4,165,184. These patents, utilizing other than the batch processing for producing the recycle mix, protect the reclaimed material in a heating drum by exposing it only to cooler gas temperatures along the heating drum interior than those hot gases to which the virgin aggregate is exposed.